Heretofore, there is a semiconductor memory called EEPROM, which is capable of repeating reading and writing. EEPROM is a non-volatile semiconductor memory in which stored data do not disappear when power is turned off. In particular, one capable of partially or entirely rewriting data at a time is called flash memory.
There are types of flash memory, which are called NAND type and NOR type. In either case, a memory cell itself has a similar structure. There are types of the structure of the memory cell, called floating gate type and charge trap type. Both the floating gate type and the charge trap type have a transistor structure of the MIS (metal-insulator-semiconductor) type. The floating gate type has a floating gate electrode provided in a gate insulating film and stores data by retaining charges in the floating gate electrode. In contrast, the charge trap type includes a gate insulating film having a layer structure of silicon oxide film-silicon nitride film-silicon oxide film (ONO structure), in which data is stored by accumulating charges in a discrete trap that is present in the vicinity of the interface between the silicon nitride film and the silicon oxide film on the silicon substrate side and thereby varying the threshold of the transistor. There are types of the charge trap type, called SONGS (Silicon Oxide Nitride Oxide Semiconductor) type and MONOS (Metal Oxide Nitride Oxide Semiconductor) type. It should be noted that, in either the floating gate type or the charge trap type, the oxide film on the silicon substrate side is called tunnel oxide film.
Although the floating gate type was previously mainstream, the charge trap type tends to be increasingly used in recent years. One of the reasons for that is that the charge trap type has an advantage that some reduction in insulating properties of part of the tunnel oxide film does not matter much because charges are captured in the discrete trap of the silicon nitride film that is an insulation film, whereas the floating gate type requires the tunnel oxide film to have high insulating properties because charges are captured in the floating gate layer. Further, since the thickness of the tunnel oxide film itself can be reduced more in the charge trap type, a voltage for writing data can be reduced in the charge trap type as well, which is also a great advantage.
For the aforementioned reasons, etc., the charge trap type tends to be favored. However, there is a demand for further improving the data retention characteristics and the endurance to repeated writing and erasing. To deal with such a demand, Patent Literature 1 discloses that a SiOxNy thin film, formed by atomic layer chemical vapor deposition, having an intermediate composition between SiO2 and Si3N4 is provided as a trap formation layer. It is disclosed that such a configuration of the trap layer allows well-controlled formation of traps with high density to a desired depth, so that the data retention characteristics and the endurance to repeated writing and erasing can be improved, and a large difference in threshold voltage as a memory effect can be ensured, which is advantageous in multi-leveling.